My Scariest villain so far is an Illithid Warlock, having made a pact with the Eldest Aboleth. He then walked that path until lichdom, becoming an Alhoon, leading the cult of the Eldest Eye.
His overall goal is the full awakening of the eldest, to bring about a horrifying vision of paradise that only total universal destruction can bring, and has met the party several times, stolen a magical crystal of colossal translocation magic power, and when the party eventually manages to defeat him, the warlock in the party will have a Strong compulsion to pick up the eye of the eldest - as it's an artefact warlock only implement, and he's already Star-Pact with a rival star god.
Things ought to get interesting,
Ayden Mitchell
There's this guy AT CR 3. I mostly wanted to post him cause he's so cute. I mean, he's a babby wyvern.
Ian Davis
>flyby >multiattack >60ft flying >save-or-suck rider That's a pretty solid option for mid-level moon druids.
Luke Young
More relevant to your original question, there's this guy, who IMO is underscored at CR 2. Also, A fear of Polar Bears is not an irrational fear.
Ryan Rodriguez
two thumbs up.
cute as fuck too. i'd totally use that.
Asher Green
>gains two levels of exhaustion
jesus christ. the save dc isn't that high but that's still kind of silly.
Jason Bell
There's also this guy. It's Nice to see the nicer "clean your house and only ask for baubles" fey getting some play, even if it's racially bound.
Easton Miller
Thinking about an undead-themed warlock patron, like you're the thrall of a lich who gives you powers to exert his will outside of his lair. Sound interesting enough to think it all the way through?
I'm thinking the player gets more undead-like as they progress, maybe the capstone is resistance to normal damage types. Is this in a book already and I'm forgetting? Is there already a popular one in the dm guild?
Connor Moore
Next we're going to prove spoony right, that the pretty woman IS always out to kill you.